When riding or driving an animal, such as a horse, it is known to use headgear to direct the animal. For example, a horse rider controls a horse with reins that are attached to a bit, which is held on a horse's head by a bridle.
While there are many different types of bits that are used in horse riding, all bits have a mouthpiece that, in use, is positioned on the bars in the interdental region and at the corners of the horse's mouth, and a pair of sidepieces at each end of the mouthpiece that connect either directly or indirectly to the reins. The sidepieces can be shanks that are configured to lever the action of the reins on the mouthpiece, or can be bit rings that directly connect the reins to the mouthpiece. There are many different shapes of bit rings, including circular rings, Dee rings, and Eggbutt shapes.
One common bit used in horse racing is a Dee bit, in which each ring has an overall “D” shape, with the straight section arranged to act against the horse's cheeks, and the mouthpiece is jointed. The joint in the mouthpiece provides a pinching action when the rider pulls the reins that urges the mouthpiece and bit rings against the soft tissues on the corners of the horse's mouth. This means that the rider can turn or slow the horse with minimal movement of the reins because there is greater communication from rider to horse. However, a significant disadvantage of this type of bit is that significant effort on the rider's part is required to release the bit and let the horse relax on the bit. This is problematic in a horse race because the horse is disinclined to accelerate with residual pressure from the bit.